Many companies are working on the implementation of compact mobile technologies for processing natural gas into synthetic hydrocarbons. The main advantage of such technologies is their possibility of use in small and distant hydrocarbon fields. In particular, they are focused on processing associated petroleum gas (APG), a significant part of which is burned off at fields. The Fischer-Tropsch synthesis technology in a compact version will improve the utilization of APG and natural gas in small and distant fields, thus increasing the profitability of their development.
Compact reactor dimensions are driven by the need for transportation of Fischer-Tropsch synthesis plants to distant fields on various types of transport, including by road. To ensure acceptable weight and dimensional characteristics and to reduce capital costs when designing mobile plants for processing hydrocarbon gases into synthetic crude oil, the output per unit mass of the reactor must be of at least 1000 g C5+/kgreactor/day, which is usually provided with a catalyst productivity of more than 1000 kg C5+/m3cat·h.
A high productivity rate of the Fischer-Tropsch process in a compact version is usually achieved by using catalysts with a content of cobalt of at least 30%, for example, as described in WO 2008104793 A2, publ. 4 Apr. 2008; EA 014214 B1, publ. 29 Oct. 2010 and others. The catalysts are required to be activated before their use to produce synthetic hydrocarbons. The activation is preferably performed in situ in a compact Fischer-Tropsch synthesis reactor under conditions (temperature, pressure) that are within the range of the working Fischer-Tropsch process conditions.
The stable operation of compact reactors at a high productivity rate for a long time is ensured by keeping the reactors in isothermal mode. Because of a high Fischer-Tropsch reaction thermal effect of 165 kJ/mol, a high activity of used catalysts, and high volumes of processed raw materials, an intensive heat removal is necessary in order to keep the Fischer-Tropsch process at its high productivity rate in the isothermal mode, for which it is expedient to use an endothermic process with a comparable thermal effect, for example the boiling of liquid. Among the available heat transfer media that can be used at temperatures of Fischer-Tropsch synthesis reactions, water is characterized by a maximum value of specific vaporization heat. In addition, the factor of heat transfer from a wall to boiling water is higher than in single-phase water flow. Under boiling, the temperature in the cooling jacket is determined by the pressure and remains constant, strictly ensuring the isothermal mode in reaction channels, and in cooling by fluid flow, the fluid flow, as passes through the reaction channel, heats up, which leads to a decrease in a temperature drop between the wall and the flow, thereby additionally reducing the efficiency of the heat removal, and may cause a disturbance in the isothermal mode in the reactor. Therefore, it is preferable that a reactor for implementation of the method for producing synthetic hydrocarbons in a Fischer-Tropsch process in a compact version allows heat removal due to the boiling of water.
The boiling of water in the cooling jacket can occur in two modes—pool boiling and flow boiling. In the first case, vapor bubbles detached from a heat-dissipating wall and carry the heat to the flow core of heat transfer medium. This generates an additional agitation, which contributes to the flow turbulization and an increase in the factor of heat transfer from the wall. In the second case, generated vapor bubbles move through a narrow channel along the heat-dissipating wall at a speed greater or less than the speed of the liquid phase, depending on the direction of the action of Archimedes force relative to the fluid flow. In this case, as the flow of the flow core of heat transfer medium passes along the heat-dissipating wall, the vapor/liquid ratio increases, which leads to a reduction in the heat transfer factor due to a low thermal conductivity of vapor compared to liquid. The reduction in the heat transfer factor in the case of flow boiling does not allow an effective heat removal along the entire length of the reaction channel and makes it impossible to keep a Fischer-Tropsch synthesis reactor in isothermal mode at its high productivity rates. Therefore, the most effective heat removal required for keeping the Fischer-Tropsch synthesis reactor in the isothermal mode can be ensured under conditions of pool boiling of water in the cooling jacket.
The surface roughness of catalytic channels is also an important parameter for intensification of the boiling process since the generation of bubbles during boiling of heat transfer medium occurs in microscopic cavities. On a smooth surface, the number of places suitable for the emergence of a germinal bubble is limited, so the boiling begins later and is unstable, due to which the same heat flow is achieved at a higher temperature difference between the wall and the core of the flow of heat transfer medium.
A compact reactor for a Fischer-Tropsch synthesis reaction, provided by Compact GTL PLC is known, the reactor consisting of channels in which there is a gas-permeable catalyst structure, as described in U.S. Pat. No. 7,217,741B2, publ. 15 May 2007. The channels of the reactor extend between headers. The construction consists of two reactor units connected in series. The syngas hourly space velocity is in the range 1000-15000 h−1 and is selected so that water vapors do not exceed 20 mol. %. To enhance heat transfer and increase the surface area of a catalyst, corrugated foils or metal meshes are used as a substrate for the catalyst within the channels. The catalyst to be used in the proposed reactor for Fischer-Tropsch synthesis is γ-Al2O3 of specific surface area 140-450 m2/g coated with cobalt in an amount of 9-29% by the catalyst weight, wherein ruthenium, platinum or gadolinium oxide is used as a promoter at a Co/promoter ratio of 10000/1 to 10/1. The reactor consists of rectangular plates, each plate being 450 mm long and 150 mm wide and 6 mm thick. Header chambers are welded along each side, each header defining three compartments. Within each of the central compartments of the headers there are coolant tubes that extend the entire height of the reactor. The Fischer-Tropsch process runs in two stages. The reaction gas is cooled between the stages so as to condense water vapors. The conversion of carbon monoxide at the first stage does not exceed 70%. At the second stage, the conversion of the residual CO is not more than 70%. The process temperature is not higher than 210° C. The productivity of a reactor of 8 m in length is 200 barr/day, which corresponds to an output per unit mass of the reactor of 550 g C5+/kgr/day in terms of the size of the reaction channels recited in the patent.
Disadvantages of such a reactor and method for conducting a Fischer-Tropsch process in this reactor are the need for a two-stage process, which leads to a low output per unit mass of the reactor; the cooling tubes located only in the central part of the reactor and the heat removal due to the flow of water inside tubes reduce the efficiency of heat removal and may result in difficulties in keeping the reactor in isothermal operation mode. The efficiency of heat removal from the reaction channels cannot be increased by pool boiling of water in the cooling tubes in the reactor of the proposed design.
U.S. Pat. No. 9,011,788 B2, publ. 21 Apr. 2015, discloses a compact reactor unit for Fischer-Tropsch synthesis of Ceramatec Inc., consisting of tubes with aluminum inserts inside them. The insert consists of six radially extending fins contacting with the inner wall of the tube. The fins comprise cross-fins disposed towards the inner surface of the tube. This design of the internal part of the reactor allows effective removal of the heat generated by the Fischer-Tropsch reaction, from the center of the catalytic layer to the reactor walls. The Fischer-Tropsch process in such a reactor is carried out in the presence of a cobalt or iron catalyst dispersed in a microfibrous matrix, at 210-235° C. with a temperature drop inside the tube of not more than 25° C.
A disadvantage of this method is the complexity of the tube design with internal inserts and an increase in the specific quantity of metal of the reactor unit by 1.5 times compared with the classical tubular reactor. In view of the increase in specific quantity of metal, the output per unit mass of the reactor at maximum catalyst productivity of 1875 kg C5+/m3cat·h is 691 g C5+/kgr/day. In addition, the maximum temperature drop of 25° C. as indicated can lead to unstable operation of the reactor at a high productivity rate.
U.S. Pat. No. 9,199,215 B2, publ. 1 Dec. 2015, describes a highly efficient reactor provided by Ceramatec Inc., consisting of several cylindrical tubes charged with a catalyst. Each reactor tube is placed within an external pipe, which in turn is housed within the reactor shell. The design of the reactor allows two cooling loops. The primary longitudinal cooling loop passes in external pipes. Thus, the heat generated by the Fischer-Tropsch reaction is transferred to the wall of the pipes. The fluid in the second cooling loop flows within the shell and across the outside of the pipes. The flow of heat transfer medium in the second cooling loop is perpendicular to the flow in the primary loop. Internal baffles divide the reactor shell into a plurality of chambers. The use of baffles makes it possible to regulate the number and direction of flows in the second loop, thereby changing the intensity of cooling. The Fischer-Tropsch process in such a reactor also runs in the presence of a cobalt or iron catalyst dispersed in a microfibrous matrix, at a temperature of 210-235° C.
A disadvantage of this invention is the need to use an additional external pipes and internal baffles to obtain a two loop cooling system, which leads to an increases in the specific quantity of metal more than 2.4 times and an increased size of the reactor and reduces the output per unit mass of the reactor to less than 650 g C5+/kgr/day. The pool-boiling mode for water, which is maximally effective for heat removal from the reaction tubes, cannot be reached in a narrow gap between the reaction tubes and the external pipes of the primary cooling loop.
The closest technical solution to the present invention is a compact reactor (microchannel unit) provided by Velocys Inc. and a method for conducting a Fischer-Tropsch reaction using said reactor, as described in U.S. Pat. No. 9,359,271 B2, publ. 7 Jun. 2016. The microchannel units are made in the form of cubic blocks with a length of 10 meters and consist of repeating units comprising reaction channels filled with a catalyst, and cooling channels filled with water. Water is fed to the cooling channels orthogonally to the feed flow in the reaction channels. Each synthesis microchannel may have a cross section having any shape, for example, a square, rectangle, circle, or semi-circle. The thicknesses of the channels may be up to 10 mm, and the length may be up to 10 m. The microchannels for heat transfer also may have any shape having a thickness of up to 2 mm, a width of up to 3 m, and a length of up to 10 m. The Fischer-Tropsch synthesis process in this reactor is conducted, according to the presented examples, in the presence of cobalt-containing catalysts based on a silica support modified with 16 wt. % TiO2, promoted by 0.05 wt. % Re, wherein the content of cobalt ranges from 18 to 43 wt. %. The catalyst is to be pre-activated at a temperature of from 300 to 600° C. and under a pressure of from 0.1 to 10 MPa for 2-24 hours in a reducing gas medium, wherein the reducing gas can be hydrogen, gaseous hydrocarbons and their mixtures, as well as a mixture of hydrogen and nitrogen, or synthesis gas. The process of producing high-molecular hydrocarbons in the microchannel reactor in the presence of the activated catalyst is carried out using synthesis gas at an H2/CO ratio of 1.4 to 2.1, a space velocity of at least 1000 h−1, a temperature of 150-300° C., and a pressure of no more than 5.0 MPa. According to the examples, the productivity of the claimed catalyst in the microchannel reactor is 680-1530 kg/m3cat·h.
A disadvantage of this reactor is inefficient removal of the reaction heat by the flow of heat transfer medium through the cooling channels. The inefficiency of heat removal in the reactor of such a design is evidenced by a high content of nitrogen in synthesis gas, which is 16.5-35.0 vol. %, since the feedstock is usually diluted with nitrogen to prevent overheating in the catalyst bed. The intensity of heat removal is adjusted by changing the flow rate of heat transfer medium and the size of the cooling channels, which does not allow keeping the reactor in isothermal operation mode and reduces productivity due to local overheating of the catalyst bed and the corresponding decrease in the catalyst selectivity for high molecular weight hydrocarbons. The efficiency of heat removal due to boiling water in the cooling channels in a reactor of this design can be increased only in the “flow boiling” mode, which is characterized by less efficient heat removal because the generated vapor bubbles cannot leave the near-wall region, as is done in the “pool boiling” mode. This does not allow an effective production of synthetic hydrocarbons in a compact reactor of the above-indicated design. Another disadvantage of this method is the pre-activation at a temperature of 300-600° C., which is higher than the temperature of the synthesis of hydrocarbons by the Fischer-Tropsch method, which is in the range of 150 to 300° C. The activation in situ in a Fischer-Tropsch synthesis reactor requires a more expensive refractory steels. In addition, the pre-activation of a catalyst at a temperature of above 300° C. makes it impossible to ensure the isothermal mode of the reactor, which will lead to uneven regeneration of the catalyst along the length of the catalyst bed and to its unstable operation in Fischer-Tropsch synthesis of hydrocarbons, as well as to a low catalyst productivity. The activation in a separate reactor (ex situ) is fraught with technological difficulties associated with the transportation of the activated catalyst to a Fischer-Tropsch reactor and with the use of additional equipment, which also contradicts the condition of compactness of the claimed method of Fischer-Tropsch synthesis.
Another disadvantage of the proposed reactor design and the method of Fischer-Tropsch synthesis in such a reactor is an increased specific quantity of metal per the structure, which reduces the daily output per unit mass of the reactor. For example, for a reactor consisting of five reaction channels and six heat removal channels with channel sizes of both types of 56×50×2 mm and a wall thickness of 2 mm, the daily productivity of the reactor in accordance with the productivity specified in the examples can be in the range of 457 to 1028 g C5+/day. In this case, the approximate mass of the reactor will be at least 1.2 kg, which corresponds to a daily output per unit mass of the reactor of 380 to 857 g C5+/kgr·day.
The technical problem of the claimed group of inventions consists in developing a compact reactor for the production of synthetic hydrocarbons in a Fischer-Tropsch process, a method for activating a cobalt catalyst, and implementing the Fischer-Tropsch process to produce synthetic hydrocarbons in the compact reactor with a high yield of synthetic hydrocarbons.